let sq = 'abcdefghijklmnopqrstuvwxyz'
let index = 26*26
let str = index==0 ? sq.charAt(0) : ''
for(let num = index; num !=0; num = Math.floor(num/sq.length)){
    let last = num%sq.length 
    str = sq[last]+str
}
console.log(str)